The present invention relates to apparatus and methods of multi-axis internal spinal fixation. In more detail, the present invention relates to a connection for use in an internal spinal fixation system, and a method of stabilizing, or fixing, the spine for use with either bilateral rods or plates (such as the Steffee/variable screw placement (VSP) system) or a central rod and plurality of cross-bars or plates (such as the so-called Tacoma Monorail System), utilizing wedge-shaped and/or flat washers having concave surfaces and, optionally, off-set and/or centered openings therein to provide multiple axes for transfering load from the patient""s spinal column to the stabilizer through the pedicle screws used to fix the rods, cross-bars, and/or plates to the vertebrae of the patient.
There are many systems available for internal fixation of the spine. Such systems are described in the patent literature (see, for instance, U.S. Pat. Nos. 4,696,290, 5,047,029, 5,092,866, 5,129,899, 5,201,734, 5,312,404, 5,531,747, and 5,743,907 and European Application No. EP 0 846 444 A1) and the scientific literature (see, for instance, D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine (Philadelphia: Nanley and Belfus, Inc.) 1992 and H. S. An and J. M. Cotler (Eds.), Spinal Instrumentation (Baltimore: Williams and Wilkins) 1992), and are available from such vendors as AcroMed, Smith and Nephew, MOSS(copyright) Miami, Osteonics, Sofamor Danek, and others.
A problem with all such systems, however, is the connection between the screws used to affix the system to the pedicle and the rods, cross-bars, and/or plates of the system. As stated in J. M. Cotler, et al., Principles, Indications, and Complications of Spinal Instrumentation: A Summary Chapter, in H. S. An and J. M. Cotler, Spinal Instrumentation pp. 435-456 (Baltimore: Wiliams and Wilkins) 1992, xe2x80x9c[a] significant problem in pedicular screw fixation appears to be at the site of linkage between the screw and rod or plate.xe2x80x9d
It appears that the problems at the site of this linkage may result from the geometry of the connection between the screw and the rod or plate. This difficult geometry results from several factors, including the different angles of the pedicles of the vertebrae, the location of the vertebrae and their relative sizes, the shape of the vertebrae and the spacing between vertebrae, the placement of the screws, the lordosis of the spine, and the need to insert the screws into each vertebra at an angle. With regard to the angle of the pedicle screws, pedicle screws are angled inwardly and upwardly into the vertebra for maximum strength and, because the surfaces of the pedicles of each vertebrae are angled relative to each other, the screws rarely line up across the vertebral body into which they are screwed. Nor do they usually line up from one vertebra to the adjacent vertebra, even if the adjacent vertebrae are the same size and shape (which they generally are not). For a more complete, discussion of the biomechanics of the bone-implant interface, reference is made to H. A. pool and R. W. Gaines, Biomechanics of Transpedicular Screw Spinal Implant Systems, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 37-44 (Philadelphia: Nanley and Belfus, Inc.) 1992, M. R. Pinto, Complication of Pedicle Screw Fixation, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 45-54 (Philadelphia: Nanley and Belfus, Inc.) 1992, and M. H. Krag, Vermont Spinal Fixator, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 121-145 (Philadelphia: Nanley and Belfus, Inc.) 1992, which references are incorporated herein in their entirety by these specific references thereto. A rod (or rods depending upon the particular stabilizer utilized) running along the longitudinal axis of the patient""s spinal column provides the structural rigidity required for the stabilizer to stabilize the spine. Because the pedicle screws do not line up, the rod(s) must either be bent to the location of each screw head or structure must he provided that can be adjusted and/or positioned to enable the head of the pedicle screw to contact the rod(s) to transfer load from he screw to the rod.
As a result of this difficulty, the literature includes comments such as the following tatement in R. M. Puno and J. A. Byrd III, Transpedicular Screw/Rod Fixation Using the Puno-Winter-Byrd (PWB) System, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 83-106 (Philadelphia: Nanley and Belfus, Inc.) 1992:
xe2x80x9cTranspedicular fixation has been proved to be of value in the treatment of spinal disorders . . . However, experience has shown that this method of instrumentation places great demand on the surgeon""s skill because of the anatomic constraints related mainly to the anatomy and morphometry of the spinal pedicle.xe2x80x9d
Many of the above-listed systems, and many of the systems described in the literature, attempt to relieve this burden on the surgeon by providing angled screws (for instance, the AMSET(copyright) R-F reduction-fixation system), so-called polyaxial screws (available from MOSS(copyright) Miami), full-length, scalloped, open-slot plate designs with an undersurface complementary to the shape of the screw head for positioning of the screws and up to 15xc2x0 medial-lateral and 30xc2x0 craniocaudal angulation at the screw-plate interface, and infinitely variable couplers (the so-called Rogozinski spinal rod system, for exanple) that are said to allow rotation through a 130xc2x0 arc to allow screw placement within the pedicle without requiring that each screw be aligned with the screw in the adjacent vertebrae.
Although they address these problems, as evidenced by the introduction of new systems by the same vendors marketing the above-listed systems, no system completely solves all the problems presented by the need for optimal screw placement, angulation of the screw, and effective load transfer from spinal column to stabilizer. An ideal system would (a) accomodate optimal screw placement, height, and angulation, (b) accomodate different sizes and shapes of vertebrae, (c) minimize (or not require) bending or other fabrication during surgery, (d) maintain an angle of approximately 90xc2x0 at the connection between the screw head and the plate or cross-bar to which the screw is attached for optimal load transfer and to minimize the likelihood of slippage and/or gross failure, and (e) be strong enough to provide lasting and rigid fixation of the spine. Those skilled in the art will recognize that this list is not exhaustive, but instead illustrates some of the characteristics of an ideal internal fixation system. Other design criteria are also important, and some practicioners may consider some criteria so important that they might not even list others.
So far as is known, none of the above-listed internal fixation systems meets these criteria in every patient. The disadvantages and limitations of currently available systems are made clear from reports in the literature of failure rates (failure of the device, not such complications as infection, phlebitis, seroma, neurologic deficit, etc.) as high as 25% (see R. Roy-Camille, et al., 203 Clin. Orthop. 7 (1986)), 11% (see, S. F. Heim and E. R. Luque, Danek Plaste and Screw System, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 201-234 (Philadelphia: Nanley and Belfus, Inc.) 1992), 8% (see, R. M. Puno and J. A. Byrd III, Transpedicular Screw/Rod Fixation Using the Puno/Winter/Byrd (PWB) System, supra), and 2-7% D. M. Arnold and L. L. Wiltse, The Wiltse System of Internal Fixation for the Lumbar Spine, in D. M. Arnold and J. E. Lonstein (Eds.), 6 State of the Art Reviewsxe2x80x94Spine: Pedicle Fixation of the Lumbar Spine 55-82 (Philadelphia: Nanley and Belfus, Inc.) 1992).
The currently available systems have other limitations. By way of example, so far as is known, no currently available surgically implanted system can predictably treat rotoscoliosis. Further, no currently available system is conveniently used in multiple level surgery. Multiple level surgery is a challenge for the surgeon because of the need to align the pedicle screws in multiple vertebrae while working under the heavy muscles of the back.
There is therefore a need for improvement of such systems, and it is this improvement to which the present invention is directed. In particular, it is an object of the present invention to improve the screw-plate interface in those systems in which the screw is angled and/or spaced at varying intervals. Another object of the present invention is to provide flexibility of placement, angulation, spacing, and screw height for accomodating the pedicle screws of such systems. Another object of the present invention is to provide a load transfer system that is universal in the sense that, although comprised of relatively few parts, it works with pedicle screws and laminar hooks, thereby providing even more flexibility and ease of use. Another object of the present invention is to provide an internal spinal fixation system that avoids the need for surgery under the heavy muscles of the back so that implantation is simplified and there is more room for fusion of adjacent vertebrae in the lateral gutter. Other objects, and the advantages, of the present invention will be made clear to those skilled in the art by the following description of the preferred embodiments thereof
These, and other objects, of the present invention to be made clear by the following detailed description of the invention, are met by providing a connection between a spinal stabilizer and a pedicle screw with a hemispherical head comprising a washer defining means for engaging a spinal stabilizer, means on the spinal stabilizer for engaging the washer, the engaging means on the washer and the engaging means of the spinal stabilizer cooperating to engage each other at a plurality of points within a common plane. The washer defines a concave surface, a bearing surface, and a passage extending through the washer for receiving the pedicle screw therethrough with the hemispherical head bearing against the concave surface of the washer when a nut bears against the bearing surface when the spinal stabilizer is affixed to a vertebral body. In one embodiment, the engaging means on the washer comprises means for resting on and rotatably engaging in the spinal stabilizer adjacent the periphery of an aperture in the spinal stabilizer so that the washer is capable of being rotated in the aperture, the combination of the concave surface and the different angles and positions of the screw providing an infinite variety of angles and pedicle screw placements while maintaining an optimal interface between the head of the screw and the washer so as to effectively transfer the load from the spinal column to the spinal stabilizer.
The invention also contemplates a spinal stabilizer including such a connection. The spinal stabilizer comprises an elongate member adapted to be affixed to a vertebra and defining a planar aperture. A washer is provided with engagement means adapted to engage the elongate member adjacent the periphery of the aperture at any of a plurality of relative rotational positions between the washer and the elongate member about a notional rotational axis that extends through the aperture, the washer having a passage therethrough, one end of the passage being located at a concave surface formed on the washer and communicating with the plane of the aperture and the other end of the passage being located at a bearing surface formed on the washer. A nut that is threaded onto a pedicle screw with a hemispherical head engages the bearing surface when the screw extends through the passage and the aperture to engage the vertebral body of the vertebra, the axis of the passage intersecting the plane of the aperture at a first angle and the bearing surface at a second angle, at least one of the angles being acute, the plane of the aperture being inclined relative to the bearing surface so that the angle of inclination of the screw extending through the passage is adjusted in dependence upon the relative rotational position between the washer and the elongate member and the hemispherical head of the pedicle screw and the concave surface of the washer providing effective transfer of the load from the patient""s spinal column to the spinal stabilizer regardless of the angle of inclination.
In another aspect, the present invention contemplates a novel washer for use in connection with an internal spinal stabilizer that comprises a cylindrical body with a passage through the body which may optionally be offset from the center of the longitudinal axis of the washer. The passage receives a pedicle screw of a type known in the art having a hemispherical head and threads for receiving a nut for affixing an internal spinal stabilizer to the vertebral body of a patient. The body of the washer is provided with a bearing surface for the nut and a concave surface for engaging the hemispherical head of the screw. Means is formed on the body for rotatably engaging the spinal stabilizer to allow the body to rotate around the 360xc2x0 of the hole to provide infinite variability in the angle and location of the interface between the screw and the plate, engagement of the hemispherical head and the concave surface, thereby providing effective transfer of the load from the spinal column to the spinal stabilizer regardless of the angle and location of the screw relative to the spinal stabilizer.
In another aspect, the present invention comprises a washer for use with an internal spinal stabilizer comprising a body defining means adapted to engage cooperating engagement means on a spinal stabilizer and a concave surface for engaging the hemispherical head of the screw with which the spinal stabilizer is affixed to the spinal column, the engagement means on the washer being adapted to engage the cooperating engagement means on the stabilizer at a plurality of points within a common plane and with the washer in any one of a plurality of relative rotational positions about an axis substantially perpendicular to the common plane relative to the stabilizer, a passage extending through the washer and defining an axis that intersects the common plane at a first predetermined angle at one end of the passage and a bearing surface against which the nut that threads onto the pedicle screw bears, the axis of the passage intersecting the plane of the bearing surface at a second predetermined angle, at least one of the first or second predetermined angles being an acute angle.
The present invention also provides a method of affixing a spinal stabilizer to the vertebra of a patient, the stabilizer comprising a washer with a bearing surface and a concave surface and having a passage therethrough, a cross-bar, and a pedicle screw having a hemispherical head and threads for receiving a nut thereon, comprising the steps of engaging the cross-bar with the washer, inserting the screw through the passage in the washer and affixing the screw to the vertebral body, and tightening the nut on the screw, the concave surface of the washer engaging the hemispherical head of the pedicle screw when the nut is tightened against the bearing surface to effectively transfer the load from the vertebra to the spinal stabilizer at any of a plurality of angles relative to the cross-bar.
It is also an aspect of the invention to proved a spinal stabilizer for affixing to the vertebral body of a patient comprising first and second elongate members attached to each other at an angle of approximately 90xc2x0, the second elongate member being rotatable about its longitudinal axis relative to the first elongate member, a washer, and means on the second elongate member for engaging the washer in any one of a plurality of relative rotational positions at a plurality of points within a common plane about an axis substantially perpendicular to the common plane.